Question: Which of these numbers is prime? ${32,\ 45,\ 65,\ 71,\ 99}$
Solution: A prime number has exactly two factors: 1 and itself. The factors of 32 are 1, 2, 4, 8, 16, and 32. The factors of 45 are 1, 3, 5, 9, 15, and 45. The factors of 65 are 1, 5, 13, and 65. The factors of 71 are 1 and 71. The factors of 99 are 1, 3, 9, 11, 33, and 99. Thus, 71 is a prime number.